The proteases are the enzymes which cleave peptide bonds in the proteins, and a number of the proteases have been found in animals, plants and microorganisms. They are used not only as reagents for research works and medical supplies, but also in industrial fields such as additives for detergents, food processing and chemical synthesis utilizing the reverse reactions, and it can be said that they are very important enzymes from an industrial viewpoint. For proteases to be used in industrial fields, since very high physical and chemical stabilities are required, in particular, enzymes having high thermostabilities are preferred to use. At present, proteases predominantly used in industrial fields are those produced by bacteria of the genus Bacillus because they have relatively high thermostability.
However, enzymes having further superior properties are desired and activities have been attempted to obtain enzymes from microorganisms which grow at high temperature, for example, thermophiles of the genus Bacillus. 
On the other hand, a group of microorganisms, named as hyperthermophiles, are well adapted themselves to high temperature environments and therefor they are expected to be a source supplying various thermostable enzymes. It has been known that one of these hyperthermophiles, Pyrococcus furiosus, produces proteases [Appl. Environ. Microbiol., volume 56, page 1992-1998 (1990), FEMS Microbiol. Letters, volume 71, page 17-20 (1990), J. Gen. Microbiol., volume 137, page 1193-1199 (1991)].
A hyperthermophile belonging to the genus Pyrococcus, Pyrococcus sp. Strain KOD1 is reported to produce a thiol protease (cysteine protease) [Appl. Environ. Microbiol., volume 60, page 4559-4566 (1994)]. Bacteria belonging to the genus Thermococcus, Staphylothermus and Thermobacteroides, which are also hyperthermophiles, are known to produce a protease [Appl. Microbiol. Biotechnol., volume 34, page 715-719 (1991)].